There are many methods of making printed elements in which printing liquids are applied to a substrate to form an image. The term "printing liquid," as it is commonly understood in the art, means a colorant in a liquid media, as distinguished from solid and dry colorants, and includes paint, toners, inks, etc. The liquid media may be an organic solvent ("solvent based") or water ("aqueous based"). The colorant may be a dye or a pigment. Other ingredients typically are present in the printing liquid, depending upon the particular printing technique being employed.
Exemplary methods of using printing liquids include gravure and press printing, xerographic techniques using liquid toners, and ink jet printing, to name but a few. Of these methods, ink jet printing has become increasingly popular, particularly for so-called "desk-top publishing" applications, because of its ability to make multi-color prints by introducing three or four primary inks on a substrate in a single pass. Other printing methods generally require at least one pass through the printer for each primary color.
Despite the breadth of techniques available for making printed elements, a common problem can arise when a multi-colored element is desired in which a printing liquid of one color is placed in abutting relationship to a printing liquid of another color. This problem is manifested in a mixing or "bleeding" of the two printing liquids at their interface, whereby the line of demarcation between the two printing liquids is obscured. Bleeding may cause undesired color formation at the interface and a concurrent loss of resolution, color separation, edge acuity and color purity in the image. The more contrasting the two adjacent liquids are in color (such as black and yellow), the more visual the bleed. Bleed is also particularly noticeable when the mixing of two inks produces a secondary color, such as when blue and yellow mix to produce green.
Bleed is a particular problem in ink jet printing because the relatively low viscosity inks used therein tend to spread and because ink jet printers have the capability of printing three or four primary colors in simultaneous (or near simultaneous) fashion.
Several methods have been proposed to prevent bleed of adjacent printing liquids. The most obvious method is to apply the two printing liquids at a distance from one another such that no intermingling or mixing of the printing liquids can occur. This method is not a solution to the problem, however, and produces images having poor resolution.
Another method, and the one most commonly used, involves delay in applying the second printing liquid until the first printing liquid is completely dry. This method is also disadvantageous, not only because of its inefficiencies, but also because it is not particularly effective. For example, it has been observed that bleed may occur even if the first printing liquid is dry, which is believed to be caused by the colorants of the first printing liquid becoming "redissolved" in the liquid medium of the second printing liquid. Thus, the more soluble the components of the first printing liquid in the liquid medium, the more likely bleed will occur even if the first printing liquid is dry. This method is particularly disadvantageous in ink jet printing applications because it places an arbitrary limitation on the efficiency of generating multi-colored prints.
U.S. Pat. No. 5,091,005 teaches that the addition of formamide to the inks will reduce the occurrence of bleed in some circumstances. Yet another approach to control bleed is to increase the rate of penetration of the printing liquid into the substrate, which has its own shortcomings. First, it is inherently limited to those printing applications using particular printing liquid/substrate combinations. For example, highly absorbant substrates may be required to control bleed. Second, bleed will still be apparent unless the first printing liquid becomes bound to the substrate such that it will not be dissolved by the liquid medium of the second printing liquid. Third, known ways of increasing penetration have disadvantages in that they have a tendency to degrade text quality.
A combination of the above approaches is disclosed in U.S. Pat. No. 5,116,409, which discloses use of zwitterionic surfactants or non-ionic amphiphiles in concentrations above their respective "critical micelle concentration." The formation of micelles containing dye molecules is said to prevent the dye molecules in each ink from mixing.
U.S. Pat. No. 5,181,045 teaches a method of ink jet printing wherein one of inks contains a dye that becomes insoluble under defined pH conditions and the other ink has a pH that renders the dye contained in the first ink insoluble. This method is inherently limited, however, to a specific group of dyes as colorants. In addition, the ink formulations are also constrained by the need for pH buffers, for example, which further limits the utility of that method.
Dyes are a common colorant used in ink jet printing due to their solubility in water. In addition, dyes provide vibrant chromatic colors on plain paper. Unfortunately, however, many dyes possess poor resistance to light, water and handling on paper. Consequently, dye colorants have deficiencies for archiving print samples.
Pigment colorants have been used as an alternative for dyes since they generally possess excellent light and water fastness. However, most pigments do not achieve the same color intensity (i.e., "chroma") on plain paper as dyes.
One approach to improve pigment chroma is to employ a vehicle that holds the colorant on the paper surface rather than allowing the pigment to diffuse into the paper. Such vehicles tend not to penetrate into the paper, however, and are not adapted for quick drying or bleed control.
Another approach to improving pigment chroma is to use a specially coated media which helps keep the colorant on the surface of the paper. However, such media typically is more expensive than paper. Furthermore, one is restricted to printing only with the special media.
Accordingly, there is a need for an improved method for printing multi-colored images that does not present the bleed problem discussed above. Moreover, there is a particular need for such an improved method that achieves the favorable color chroma that may be obtained with dye colorants on plain paper, while providing the excellent resistance to water and light obtainable with pigment colorants.